This invention relates to a method of making a PIN photodiode device and more specifically to a method of making PIN photodiode devices which can operate at high voltages without premature breakdown.
Typically, a PIN photodiode device will comprise a silicon semiconductor body having an incident surface and a surface opposite the incident surface. The body includes a region of intrinsic semiconductor material of high resistivity i.e., 5,000 to 30,000 .OMEGA.-cm, at a portion of the incident surface, a contacting region at the opposite surface and in contact with the intrinsic region, and an incident region in the intrinsic region at the remaining portion of the incident surface. Electrical contact of one polarity is made to the device at the contacting region and light impinges the device where the incident region is at the incident surface. The contacting region is of a conductivity type opposite to that of the incident region. The intrinsic region is of a very low dopant density, i.e., in the range of 5 .times. 10.sup.11 to 5 .times. 10.sup.13 atoms/cm.sup.3, but is of a conductivity type opposite to that of the incident region. Thus, between the incident and intrinsic regions is a junction providing the device its diode effect.
In the operation of a PIN photodiode, a reverse bias voltage is placed across the body at the incident and contacting regions, so as to create a depletion region throughout the intrinsic region of the body. A depletion region is a region depleted of electrical carriers. The depletion region provided in the intrinsic region makes it sensitive to light absorption. Any light incident onto the PIN photodiode at the incident surface, traversing the intrinsic region and absorbed in the intrinsic region will generate an electron-hole pair. The generated electron and hole carriers are swept by the electric field of the depletion region to either the incident or contacting regions, depending on their conductivity type. It is this diffusion of the generated carriers across the junction of the incident and intrinsic regions which is detected as a current from the PIN photodiode device.
Either the incident or contacting region will be of an N-type conductivity and typically, the region of N-type conductivity is formed by depositing a phosphorus silicate glass (PSG) on a body of intrinsic semiconductor material in a deposition furnace; allowing the PSG to densify by leaving it in the furnace in an inert or forming gas ambient; and then diffusing the phosphorus atoms of the PSG into the body by leaving the PSG coated body in the furnace for an extended period of time. Typically, the deposition step takes from 5 to 15 minutes at a furnace temperature of 800.degree. to 1,000.degree. C; the densification step takes about 2 to 15 minutes at 800.degree. to 1,000.degree. C; and the diffusion step takes about 20 to 100 minutes at a furnace temperature of 800.degree. to 1,000.degree. C.
The reason diffusion must be from a source of N-type dopant which has been deposited on the intrinsic semiconductor body i.e., diffusion from solid to solid, and not by a gas diffusion technique, is that the dopant concentration to be deposited on a surface of the device and then diffused into either the incident or intrinsic region is high. High dopant diffusion, in the order of 10.sup.19 atoms/cm.sup.2, can not be accomplished by gas to solid diffusion. The dopant concentration is high in both the contacting and incident regions in order to maintain these regions at a low resistivity, i.e., about 5 to 50 .OMEGA.-cm. The low resistivity of the incident and intrinsic regions is needed so as to prevent the depletion region from entering either of these regions, thereby preventing any carriers from being injected into the intrinsic region from the contacting and incident regions. Also, the low resistivity and high carrier concentration results in a very short life time for any electrical carriers in the contacting or intrinsic regions, thus dark current leakage due to carrier injection from surface states from the contacting and incident regions are reduced.
If light of a long wavelength, i.e., 1 micron or longer, is to be detected by the PIN photodiode device and if the intrinsic region is of a semiconductor material such as silicon, then the intrinsic region will have to be thick, i.e., from 10 to 40 mils in thickness, to assure absorption of some of the incident light in the intrinsic region. It is well known in the photodiode field that silicon is not very sensitive to incident light of a long wavelength. Thus, the thicker the silicon intrinsic region the more light will be absorbed.
As a result of the requirement of a thick intrinsic region, the PIN photodiode must be reversed biased at a high voltage, typically in the range of 150 to 400 volts, to assure depletion of the entire intrinsic region. The thick intrinsic region and the high operating voltage of the PIN photodiode device accomplishes the two objectives of the device. These objectives are to detect as much of the light impinging the photodiode at the incident surface as possible, and for the photodiode to respond as fast as possible. The thick intrinsic region increases the quantum efficiency of the device by increasing the absorption of light, and the high operating voltage makes possible a fast response time in addition to assuring depletion of the entire intrinsic region.
If, in addition to the higher voltage requirement, the PIN photodiode device has an incident surface having a large area on which light impinges i.e., 0.5 to 3 cm.sup.2, or if a photosensing device consists of a plurality of PIN photodiodes on a single body, there is an extremely high incidence of premature voltage breakdown. The cause of the high failure rate in PIN photodiode devices of a large incident surface area, operating at a high voltage has troubled those in the PIN photodiode art. The discovery of what is causing this problem and its solution would be most desirable in the PIN photodiode field.